Reinforcing fibres properties

Fig. 26.2. The microstructure of wood. Woods ore foams of relative densities between 0.07 and 0.5, with cell walls which ore fibre-reinforced. The properties ore very anisotropic, partly because of the cell shape and partly because the cell-wall fibres ore aligned near the axial direction.

Reinforcing fibres have diameters varying from 7 /im to 1(X) /im. They may be continuous or in the form of chopped strands (lengths 3 mm-50 mm). When chopped strands are used, the length to diameter ratio is called the Aspect Ratio. The properties of a short-fibre composite are very dependent on the aspect ratio - the greater the aspect ratio the greater will be the strength and stiffness of the composite. 

Table 3.3 indicates the extent to which the properties of plastics are influenced by the level of fibre content. Full details of the forms in which reinforcing fibres are available for inclusion in plastics are given in Chapter 4. 

Kevlar has the highest tensile strength and is often used as a reinforcing fibre in composites with, e.g., epoxy, PEEK. The thermotropic liquid crystal polymer Vectran is made by melt polymerisation of p-acetoxybenzoic acid and 6-acetoxy-2-naphthoic acid, (the corresponding hydroxy acids decompose on melting). Because of its liquid crystal properties the polymer can be spun into fibres from the melt. Kevlar is spun from a solution in concentrated sulfuric acid, and can be melt drawn to give a high modulus (stiff) polymer. Vectran ... 

Table 6.30 shows the properties of the same thermoplastic (short fibre reinforced polyamide) reinforced with the same level of the three main reinforcement fibres, illustrating the effect of the fibre nature. 

The properties of fibre-reinforced composites are for example determined by a) length, diameter, orientation, volume fraction and properties of the fibres, b) the properties of the matrix and c) the nature of the bond between fibre and matrix. The ratio between length and diameter is an important fibre property. A small diameter means that the area is also small and consequently there is less chance of defects occurring in that area and so less chance of the fibre breaking. 

Polyether ether ketone (PEEK) and Polyether sulphone (PES) belong to the most recent developments in the field of technical high-performance polymers. Both possess very good thermal and mechanical properties, which can be further improved by reinforcing fibres. Their application is mainly in aircraft and space vehicles. 

Janes, Neumann and Sethna ° reviewed the general subject of solid lubricant composites in polymers and metals. They pointed out that the reduction in mechanical properties with higher concentrations of solid lubricant can be offset by the use of fibre reinforcement. Glass fibre is probably the most commonly used reinforcing fibre, with carbon fibre as a second choice. Metal and ceramic fibres have been used experimentally to reinforce polymers, but have not apparently been used commercially. To some extent powders such as bronze, lead, silica, alumina, titanium oxide or calcium carbonate can be used to improve compressive modulus, hardness and wear rate. 

Like other composite properties, the wear resistance is influenced by the orientation of reinforcing fibres. Sung and Suh found that with biaxially-oriented glass fibre and molybdenum disulphide in a PTFE matrix (Duroid 5813), the wear resistance was greatest when the highest proportion of fibre was normal to the sliding surface. The same effect was found with a graphite fibre/epoxy composite and a Kevlar fibre/epoxy composite. 

This paper reviews recent work upon structure/mechanical-property relationships in polydiacetylenes. It is shown how this has led to the development of high strength polydiacetylene single crystal fibres and their performance as reinforcing fibres in composites is described. 

Physical and thermal properties Bakelite is a thermoset, hard, brittle material. Its high shrinkage on moulding and brittleness are mediated by adding fillers or reinforcing fibres. It resists burning. 

Superior mechanical properties, including impact strength and resistance to delamination, are claimed for reinforced plastics made from three-dimensional fabrics [1], Other reinforcement forms include continuous swirl mat, designed to have a minimum of fibre ends, and hybrid fabrics containing two different reinforcing fibres, for example, glass and aramid. There are many different ways in which two different fibres can be combined together. 

Reinforced plastics, sometimes called polymer composites, consist of reinforcing fibres or particles embedded in a polymeric matrix. It is now recognized that a third component, called an interphase region, can exist at the interface between the fibre and resin. The properties of the interphase are probably not constant but vary to give a graded region. Thus the environmental durability of a composite material is a complex interplay between the various microstructural aspects of the material, which are ... 

Polyethylene fibres are made by several different processes and their thermal characteristics vary, but lack of high temperature resistance is very noticeable compared with other reinforcing fibres. Most polyethylene fibres begin to lose their mechanical properties at about 120°C and they melt soon afterwards. 

Figure I. Comparison of the properties of different natural reinforcement fibres.

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